The present invention relates, in general, to level sensing, and more particularly to electro-optical devices for measuring the distance between a reference point and a variable surface such as the surface level of a liquid.
In the measurement of the depth of a liquid below a reference level, and in particular in the measurement of liquids such as molten glass, liquid chemicals of various kinds, fuels such as gasoline, kerosene and the like, and in many other hazardous measurement situations, it has been found desirable to utilize non-contacting methods of measurement. Although the prior art contains numerous contact and nucleonic methods and devices for level measurement, problems of accuracy, repeatability, and sensitivity have been encountered in the measurement of liquid levels, where high temperatures, surface disturbances caused by vibrations, and the like create serious difficulties and result in generally unsatisfactory measurements.
Such problems have been solved in part by optical systems of the type wherein a laser beam is directed at an angle to the glass surface, and vertical displacement of the reflected beam caused by changes in the glass surface position is detected. The detection of laser beam displacement may be achieved, for example, by the use of a geometric photodetector in which variations in the position of the reflected light spot on the detector surface act to change the resistance of the detector. In such devices, the geometric photodetector may be incorporated in a bridge configuration as part of a null balance servo so that movement of the laser spot on the detector due to a change in level of the surface being measured creates an imbalance in the bridge and causes the servo to drive to a new null position. Such motion can be translated electrically to display the new level. Another method of detecting such displacement is disclosed in U.S. patent application Ser. No. 235,400 of Charles King, filed on Feb. 18, 1981 and entitled "Optical Measuring System". That application teaches the use of a curved mirror which directs the reflected light spot onto a rotating mirror which, in turn, directs the spot onto a photodetector when the rotating mirror is at the correct angular position. A displacement of the spot changes the angular position of the rotating mirror at which the spot will be directed onto the photodetector, so that the angular position is a measure of the surface level.
Although such measurement devices have worked well in certain applications, it has been found that they are effectively limited to relatively small level excursions, and in addition, are sensitive to surface irregularities caused by waves or debris, as well as to shifts in the supporting structure, due to barometric pressure changes, ambient temperature changes, and the like. Accordingly, a real need exists for an electro-optical level sensor having a broad range of measurement, greater flexibility in its application to various measurement tasks, reduced maintenance requirements and reduced sensitivity to structural shifting and vibration.